Flash memory device and programming and erasing methods therewith

ABSTRACT

A flash memory device and programming and erasing methods therewith is disclosed, to secure the programming and erasing characteristics by changing a structure of a floating gate, in which the flash memory device includes a first conductive type semiconductor substrate defined as a field area and an active area; a tunnel oxide layer on the active area of the first conductive type semiconductor substrate; a floating gate on the tunnel oxide layer, having at least first and second floating gates having different levels of energy band gap; a dielectric layer on the floating gate; a control gate on the dielectric layer; and second conductive type source/drain regions in the active area of the first conductive type semiconductor substrate at both sides of the floating gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Application Nos.P2003-101389 and P2003-101387, filed on Dec. 31, 2003, which is herebyincorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flash memory device and programmingand erasing methods therewith, and more particularly, to a flash memorydevice and programming and erasing methods therewith, to secure theprogramming and erasing characteristics by changing a structure of afloating gate.

2. Discussion of the Related Art

Among semiconductor memory devices, a non-volatile memory device is anoptimal device in that it is possible for a user to program data witheasiness by switching a memory state in an electrical method, and also,it is possible to maintain the memory state of data even if a power isswitched off.

On the fabrication process, the non-volatile memory device is largelyclassified into a floating gate type and an MIS(Metal-Insulator-Semiconductor) type, wherein the MIS type is formed oftwo or three dielectric layers.

The floating gate type non-volatile memory device realizes the memorycharacteristics by using a potential well. In detail, an ETOX (EPROMTunnel Oxide) structure of EEPROM (Electrically Erasable & Programmable& Programmable Read Only Memory) is one of the most representativefloating gate type non-volatile memory devices.

Meanwhile, the MIS type non-volatile memory device performs a memoryfunction by using traps remaining in dielectric layer, bulk, dielectriclayer-interface of dielectric layer, and dielectric layer-interface ofsemiconductor.

Programming and erasing methods of a floating gate type non-volatilememory device will be described with reference to the accompanyingdrawings.

FIG. 1 shows a cross sectional view of a memory device having an ETOXstructure among floating gate type non-volatile memory devices accordingto the related art.

In a related art flash memory device, as shown in FIG. 1, a tunnel oxidelayer 102, a floating gate 103, a dielectric layer 104 and a controlgate 105 are sequentially deposited on a p-type semiconductor substrate101. At this time, a source region S and a drain region D are formed inthe surface of the p-type semiconductor substrate 101 at both sides ofthe deposited structure.

Programming and erasing methods of the floating gate type non-volatilememory device will be described as follows.

On the programming method, electrons are injected to a potential wellformed in the floating gate, whereby a threshold voltage increases. Onthe erasing method, holes are injected to the potential well, wherebythe holes are recombined with the electrons, thereby lowering thethreshold voltage.

At this time, the electrons and the holes are generally injected in ahot electron injection method and a hot hole injection method. Onerasing, in case of using an F-N (Fowler-Nordheim) tunneling instead ofthe hot hole injection method, it has the disadvantageouscharacteristics such as low erasing-speed. In this respect, the hot holeinjection method is most generally used.

However, the related art programming and erasing methods have thefollowing disadvantages.

On programming and erasing, the hot electron injection method and thehot hole injection method are generally used, whereby trap sitesgenerate in the interface between the tunnel oxide layer and thesemiconductor substrate, or the inside of the tunnel oxide layer, or theinterface between the tunnel oxide layer and the floating gate. As aresult, it is impossible to maintain the constant threshold voltage dueto the trap sites. Also, the electrons or the holes stored in thefloating gate are discharged through the trap sites.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a flash memory deviceand programming and erasing methods therewith that substantiallyobviates one or more problems due to limitations and disadvantages ofthe related art.

An object of the present invention is to provide a flash memory deviceand programming and erasing methods therewith, to secure the programmingand erasing characteristics by changing a structure of a floating gate.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, aflash memory device includes a first conductive type semiconductorsubstrate defined as a field area and an active area; a tunnel oxidelayer on the active area of the first conductive type semiconductorsubstrate; a floating gate on the tunnel oxide layer, having at leastfirst and second floating gates having different levels of energy bandgap; a dielectric layer on the floating gate; a control gate on thedielectric layer; and second conductive type source/drain regions in theactive area of the first conductive type semiconductor substrate at bothsides of the floating gate.

In another aspect, a flash memory device includes a first conductivetype semiconductor substrate defined as a field area and an active area;a tunnel oxide layer on the active area of the first conductive typesemiconductor substrate; a first floating gate and a second floatinggate for being in contact with each other in parallel, on the tunneloxide layer; a dielectric layer on the first floating gate; a controlgate on the dielectric layer; and second conductive type source/drainregions in the active area of the first conductive type semiconductorsubstrate at both sides of the first floating gate.

At this time, the dielectric layer is formed in a structure of oxidelayer-nitride layer-oxide layer.

Also, the second floating gate has an energy band gap higher than thatof the semiconductor substrate and lower than that of the dielectriclayer.

In another aspect, a programming method of the flash memory deviceincluding a first floating gate and a second floating gate contactingwith each other on a first conductive type semiconductor substrate; acontrol gate on the first floating gate; and second conductive typesource/drain regions on the semiconductor substrate at both sides of thefirst floating gate, wherein the second floating gate has an energy bandgap higher than that of the first floating gate, and has secondconductive type impurity ions implanted thereto, includes steps ofapplying a positive (+) voltage to the control gate; applying agrounding voltage or a negative (−) voltage to the second floating gate;and floating the source/drain regions and the semiconductor substrate,whereby electrons generate in the second floating gate, and thegenerated electrons are transferred to and stored in the first floatinggate.

In another aspect, an erasing method of the flash memory deviceincluding a first floating gate and a second floating gate contactingwith each other on a first conductive type semiconductor substrate; acontrol gate on the first floating gate; and second conductive typesource/drain regions on the semiconductor substrate at both sides of thefirst floating gate, wherein the second floating gate has an energy bandgap higher than that of the first floating gate, and has secondconductive type impurity ions implanted thereto, and the first floatinggate has electrons stored therein, includes steps of injecting holes tothe first floating gate to inducement for recombination of the holeswith the electrons stored in the first floating gate, or discharging theelectrons stored in the first floating gate to the semiconductorsubstrate by an F-N tunneling method.

In another aspect, a flash memory device includes a first conductivetype semiconductor substrate defined as a field area and an active area;a tunnel oxide layer on the active area; a first floating gate and asecond floating gate for being in contact with each other in parallel,on the tunnel oxide layer; a dielectric layer formed on the firstfloating gate and the second floating gate; a control gate on thedielectric layer; and second conductive type source/drain regions in theactive area of the semiconductor substrate at both sides of the firstfloating gate/the second floating gate.

At this time, the first floating gate has a width corresponding to (orless than) a width of a depletion layer of the source/drain regions.

Also, the second floating gate has an energy band gap higher than thatof the first floating gate and lower than that of the dielectric layer.

In another aspect, a programming method of the flash memory deviceincluding a first floating gate and a second floating gate contactingwith each other in parallel, on a semiconductor substrate; a tunnelingoxide between the first and second floating gates and the substrate; acontrol gate on the first and second floating gates; and secondconductive type source/drain regions in the semiconductor substrate atboth sides of the first and second floating gates, wherein the secondfloating gate has an energy band gap higher than that of the firstfloating gate, includes steps of applying a positive (+) voltage to thecontrol gate and the drain region; and grounding the semiconductorsubstrate and the source region, whereby hot electrons generate in adepletion area of the drain region, the hot electrons are injected tothe second floating gate by the tunnel oxide layer, and the electronsinjected to the second floating gate are transferred to the firstfloating gate.

In another aspect, an erasing method of the flash memory deviceincluding a first floating gate and a second floating gate contactingwith each other in parallel, on a semiconductor substrate; a tunnelingoxide between the first and second floating gates and the substrate; acontrol gate on the first and second floating gates; and secondconductive type source/drain regions in the semiconductor substrate atboth sides of the first and second floating gates, wherein the secondfloating gate has an energy band gap higher than that of the firstfloating gate, and electrons are stored in the first floating gate,includes steps of respectively applying a negative (−) voltage and apositive (+) voltage to the control gate and the drain region; andgrounding or floating the semiconductor substrate and the source region,whereby holes generate in a depletion area of the drain region, theholes are injected to the second floating gate by the tunnel oxidelayer, and the holes injected to the second floating gate aretransferred to the first floating gate, and are recombined with thestored electrons.

In another aspect, a flash memory device includes a first conductivetype semiconductor substrate defined as a field area and an active area;a tunnel oxide layer on the active area of the first conductive typesemiconductor substrate; a first floating gate and second/third floatinggates for being in contact with each other, on the tunnel oxide layer,the second/third floating gates formed at both sides of the firstfloating gate; a dielectric layer on the first floating gate; a controlgate on the dielectric layer; and second conductive type source/drainregions in the active area of the semiconductor substrate at both sidesof the first floating gate.

At this time, the second and third floating gates are formed above thesource/drain regions.

Also, the second and third floating gates have an energy band gap higherthan that of the first floating gate and lower than that of thedielectric layer.

Also, second conductive type impurity ions are implanted to the secondfloating gate, and first conductive type impurity ions are implanted tothe third floating gate.

In another aspect, a programming method of the flash memory deviceincluding a first floating gate and second/third floating gatescontacting with each other in parallel, on a first conductive typesemiconductor substrate, the second/third floating gates formed at bothsides of the first floating gate; a control gate on the first floatinggate; and second conductive type source/drain regions formed in thesemiconductor substrate at both sides of the first floating gate,wherein the second and third floating gates have an energy band gaphigher than that of the first floating gate, and second conductive typeimpurity ions are implanted to the second floating gate, and firstconductive type impurity ions are implanted to the third floating gate,includes steps of applying a positive (+) voltage to the control gate;applying a grounding voltage or a negative (−) voltage to the secondfloating gate; and floating the source/drain regions and thesemiconductor substrate, whereby electrons generate in the secondfloating gate, and the generated electrons are transferred to and storedin the first floating gate.

In another aspect, an erasing method of the flash memory deviceincluding a first floating gate and second/third floating gatescontacting with each other in parallel, on a first conductive typesemiconductor substrate, the second/third floating gates formed at bothsides of the first floating gate; a control gate on the first floatinggate; and second conductive type source/drain regions formed in thesemiconductor substrate at both sides of the first floating gate,wherein the second and third floating gates have an energy band gaphigher than that of the first floating gate, second conductive typeimpurity ions are implanted to the second floating gate, firstconductive type impurity ions are implanted to the third floating gate,and electrons are stored in the first floating gate, includes steps ofapplying a grounding voltage or a negative (−) voltage to the controlgate; applying a positive (+) voltage to the third floating gate; andfloating the source/drain regions, the semiconductor substrate and thesecond floating gate, whereby holes generate in the third floating gate,and the generated holes are transferred to the first floating gate, andthe electrons are recombined with the electrons stored in the firstfloating gate.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 shows a cross sectional view of a memory device having an ETOXstructure among floating gate type non-volatile memory devices accordingto the related art;

FIG. 2 shows a cross sectional view of a flash memory device accordingto the first embodiment of the present invention;

FIG. 3 shows an exemplary view of explaining energy band and transfer ofelectrons in a passivation layer/a second floating gate/a first floatinggate/a passivation layer along I-I′ of FIG. 2;

FIG. 4 shows a cross sectional view of a flash memory device accordingto the second embodiment of the present invention;

FIG. 5 shows an exemplary view of explaining energy band and transfer ofelectrons in a semiconductor substrate/a tunnel oxide layer/a secondfloating gate/a dielectric layer/a control gate along II-II′ of FIG. 4;

FIG. 6 shows an exemplary view of explaining energy band and transfer ofelectrons in a passivation layer/a second floating gate/a first floatinggate/a passivation layer along II-II′ of FIG. 4;

FIG. 7 shows an exemplary view of explaining energy band and transfer ofholes in a semiconductor substrate/a tunnel oxide layer/a secondfloating gate/a dielectric layer/a control gate along II-II′ of FIG. 4;

FIG. 8 shows an exemplary view of explaining energy band and transfer ofholes in a passivation layer/a second floating gate/a first floatinggate/a passivation layer along II-II′ of FIG. 4;

FIG. 9 shows a cross sectional view of a flash memory device accordingto the third embodiment of the present invention;

FIG. 10 shows an exemplary view of explaining energy band and transferof electrons in a passivation layer/a second floating gate/a firstfloating gate/a third floating gate/a passivation layer along III-III′of FIG. 9; and

FIG. 11 shows an exemplary view of explaining energy band and transferof holes in a passivation layer/a second floating gate/a first floatinggate/a third floating gate/a passivation layer along III-III′ of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Hereinafter, a flash memory device and programming and erasing methodstherewith according to the present invention will be described withreference to the accompanying drawings.

FIRST EMBODIMENT

FIG. 2 shows a cross sectional view of a flash memory device accordingto the first embodiment of the present invention. In the flash memorydevice according to the first embodiment of the present invention, asshown in FIG. 2, a semiconductor substrate 201 is defined as a fieldarea and an active area, and a device isolation layer (not shown) isformed in the field area.

In the active area of the semiconductor substrate 201, there are atunnel oxide layer 202, a floating gate 203, a dielectric layer 204, anda control gate 205 formed in sequence. At this time, the semiconductorsubstrate 201 may be formed of n-type or p-type, wherein the p-typesemiconductor substrate will be described for convenience ofexplanation. Although not shown, a passivation layer is deposited on anentire surface of the semiconductor substrate 201 including the controlgate 205.

The dielectric layer 204 may be formed in a structure of oxidelayer-nitride layer-oxide layer. Also, the floating gate 203 and thecontrol gate 205 may be formed of polysilicon to which n-type impurityions are implanted. The floating gate 203 is comprised of a firstfloating gate 203 a and a second floating gate 203 b, wherein a width ofthe first floating gate 203 a corresponds to a width of the control gate205.

Then, n-type impurity ions are implanted into the semiconductorsubstrate 201 at both sides of the first floating gate 203 a/the controlgate 205, thereby forming a source region S and a drain region D. Thesecond floating gate 203 b is in contact with the first floating gate203 a, and the second floating gate 203 b is formed on the tunnel oxidelayer 202 for being overlapped with the source region S or the drainregion D. That is, the tunnel oxide layer 202 extends to the sourceregion S or the drain region D at a predetermined degree. At this time,a width of the second floating gate 203 b is not limited. For example,the second floating gate 203 b is formed at a predetermined minimumwidth to apply a bias-voltage thereto, and not to have effects onspacers and silicide formed in the source region S or the drain regionD.

Also, the first floating gate 203 a is formed of polysilicon. The secondfloating gate 203 b is formed of a material having an energy band (Eg)higher than that of silicon Si (Eg-1.1 eV) of the semiconductorsubstrate 201 or the first floating gate 203 a, and lower than that ofoxide layer SiO₂ of the dielectric layer 204 being in contact with thefirst floating gate 203 a. For example, the second floating gate 203 bmay be formed of any one of chemical compound semiconductors of Sic,Alp, AlSb, GaP, GaAs, InP, ZnS, ZnSe, ZnTe, CdS, CdSe and CdTe, or anyone of oxides of Al₂O₃, Y₂O₃, HfO₂, ZrO₂, BaZrO₂, BaTiO₃, Ta₂O₅, CaO,SrO, BaO, La₂O₃, Ce₂O₃, Pr₂O₃, Nd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Tb₂O₃,Db₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃ and Lu₂O₃. Also, the secondfloating gate 203 b is doped with impurity ions having the oppositeconductive type to that of the semiconductor substrate 201. For example,in case the semiconductor substrate 201 is in p-type, n-type impurityions are implanted to the second floating gate 203 b. Meanwhile, in casethe semiconductor substrate 201 is in n-type, p-type impurity ions areimplanted to the second floating gate 203 b.

A programming method using the flash memory device according the firstembodiment of the present invention will be described as follows.

FIG. 3 shows an exemplary view of explaining energy band and transfer ofelectrons in the passivation layer/the second floating gate/the firstfloating gate/the passivation layer along I-I′ of FIG. 2.

In order to program data, first, a positive (+) voltage is applied tothe control gate (Vg) 205, and a ground voltage or a negative (−)voltage is applied to the second floating gate (Vf2) 203 b. In thisstate, the source region (Vs) S, the drain region (Vd) D and thesemiconductor substrate (Vsub) 201 are floated together. At this time,it is preferable to provide the voltage applied to the control gate 205and the second floating gate 203 b in the conditions of generating agreat amount of direct electron injections.

When the voltage is applied in this conditions, electrons generate dueto the bias-voltage applied to the second floating gate 203 b since thesecond floating gate 203 b is doped with the n-type impurity ions. Then,the generated electrons are transferred to a conduction band (Ec) of thefirst floating gate 203 a having a stable energy potential. At thistime, an electric field applied to the control gate 205 extends to thesecond floating gate 203 b by the first floating gate 203 a, so thatelectrons injected from the second floating gate 203 b are stored in apotential well of the first floating gate 203 a.

The process of transferring the electrons generated in the secondfloating gate 203 b to the first floating gate 203 a will be explainedas follows.

First, the energy band gap (Eg) means the energy required fortransferring the electrons from a valence band (Ev) to the conductionband (Ec). As shown in FIG. 3, the level of the energy band gap (Eg) isin order of the first floating gate 203 a, the second floating gate 203b, and the passivation layer. That is, the energy band gap (Eg) of thefirst floating gate 203 a is high, and the energy band gap (Eg) of thepassivation layer is low.

The second floating gate 203 b is formed of the material having theenergy band gap (Eg) higher than that of the silicon Si (Eg-1.1 eV) andlower than that of the silicon oxide layer SiO₂, and the second floatinggate 203 b is in contact with the first floating ate 203 a of thepolysilicon material. As a result, the electrons of the conduction bandof the second floating gate 203 b are transferred to the conduction bandof the first floating gate 203 a which is more stable. In this state,the electrons, transferred to the conduction band of the first floatinggate 203 a by the electric field, are stably stored in the potentialwell of the first floating gate 203 a, whereby a threshold voltageincreases. Accordingly, the programming method of the flash memorydevice according to the first embodiment of the present invention iscompleted.

As described above, in the first embodiment of the present invention,the floating gate is formed of the two materials (the first floatinggate 203 a and the second floating gate 203 b) having the differentlevels of the energy band gap. Also, the second floating gate 203 b isformed of the semiconductor having the n-type impurity ions implantedthereto, and the energy band gap of the second floating gate 203 b ishigher than the energy band gap of the first floating gate 203 a.Accordingly, the electrons generate from the second floating gate 203 bby the bias-voltage applied to the second floating gate 203 b, and thegenerated electrons are transferred to the first floating gate 203 a.Then, the electrons transferred to the first floating gate 203 a arestably stored in the first floating gate 203 a as long as any voltage isnot applied from the external. As a result, the applied thresholdvoltage is maintained stably.

In the related art of injecting the electrons to the floating gate bythe tunnel oxide layer, trap site generates in the interface and theinside of the tunnel oxide layer. However, the present invention has noproblem of the trap site.

Meanwhile, like the related art erasing method, an erasing method of theflash memory device according to the first embodiment of the presentinvention uses a hot hole injection method. That is, holes are injectedto the potential of the first floating gate 203 a, so that it ispossible to induce the recombination of holes with the electrons storedin the first floating gate 203 a, thereby decreasing the thresholdvoltage. In addition to the hot hole injection method, the electronsstored in the first floating gate 203 a may be discharged to thesemiconductor substrate in an F-N tunneling method, so as to decreasethe threshold voltage.

SECOND EMBODIMENT

A flash memory device according to the second embodiment of the presentinvention will be described as follows. FIG. 4 shows a cross sectionalview of a flash memory device according to the second embodiment of thepresent invention.

In the flash memory device according to the second embodiment of thepresent invention, as shown in FIG. 4, a semiconductor substrate 201 isdefined as a field area and an active area, and a device isolation layer(not shown) is formed in the field area. At this time, the semiconductorsubstrate 201 may be formed of n-type or p-type, wherein the p-typesemiconductor substrate will be described for convenience ofexplanation.

In the active area of the semiconductor substrate 201, there are atunnel oxide layer 202, a floating gate 203, a dielectric layer 204, anda control gate 205 formed in sequence. Then, n-type impurity ions areimplanted into the semiconductor substrate 201 at both sides of thefirst floating gate 203/the control gate 205, thereby forming a sourceregion S and a drain region D. Also, although not shown, a passivationlayer is formed on an entire surface of the semiconductor substrate 201including the control gate 205.

The dielectric layer 204 may be formed in a structure of oxidelayer-nitride layer-oxide layer. The control gate 205 may be formed ofpolysilicon having n-type impurity ions implanted thereto. Also, thefloating gate 203 is comprised of a first floating gate 203 a and asecond floating gate 203 b. Herein, a width ‘d1’ of the second floatinggate 203 b is corresponding to (or less than) a width ‘d2’ of adepletion area 206 extended from the drain region D. Preferably, thewidth ‘d1’ of the second floating gate 203 is determined at a degreebetween 400 Å and 600 Å. Also, the first floating gate 203 a is formedof polysilicon. The second floating gate 203 b is formed of a materialhaving an energy band gap (Eg) higher than that of silicon Si (Eg-1.1eV) of the semiconductor substrate 201 or the first floating gate 203 a,and lower than that of oxide layer SiO₂ of the dielectric layer 204being in contact with the first floating gate 203 a. The second floatinggate 203 b is formed of the same material as that according to the firstembodiment of the present invention.

Programming and erasing methods using the flash memory device accordingto the second embodiment of the present invention will be described asfollows.

FIG. 5 shows an exemplary view of explaining energy band and transfer ofelectrons in the semiconductor substrate/the tunnel oxide layer/thesecond floating gate/the dielectric layer/the control gate along II-II′of FIG. 4. FIG. 6 shows an exemplary view of explaining energy band andtransfer of electrons in the passivation layer/the second floatinggate/the first floating gate/the passivation layer along II-II′ of FIG.4.

In order to program data, first, predetermined positive (+) voltages(Vg, Vd) are respectively applied to the control gate 205 and the drainregion D, and the source region (Vs) S and the semiconductor substrate(Vsub) 201 are grounded. At this time, preferably, the voltages (Vg, Vd)applied to the control gate 205 and the drain region D have the optimalconditions of generating a great amount of hot electron injections.

On applying bias, electrons of the source region S are transferred to achannel region below the tunnel oxide layer 202. Then, the electrons areaccelerated by an electric field applied in horizontal, whereby hotelectrons generate around the drain region D. After that, when the hotelectrons are transferred toward the tunnel oxide layer 202 by avertical electric field formed by the positive (+) voltage applied tothe control gate 205, the hot electrons get over an energy barrierbetween the semiconductor substrate 201 and the tunnel oxide layer 202.Thereafter, the hot electrons are injected to a conduction band (Ec) ofthe second floating gate 203 b.

The process of transferring the electrons from the channel region of thesemiconductor substrate 201 to the second floating gate 203 b will bedescribed with reference to FIG. 5. Referring to FIG. 5, in statematerial layers are formed in order of the semiconductor substrate/thetunnel oxide layer/the second floating gate/the dielectric layer/thecontrol gate, the energy required for being transferred to the energyband gap (Eg) to each material layer is shown.

As shown in FIG. 5, the level of energy band gap (Eg) is in order ofconductor, semiconductor and insulator, wherein the energy band gap (Eg)of the conductor is high, and the energy band gap (Eg) of the insulatoris low. To transfer the electrons remaining in the channel region of thesemiconductor substrate 201 to the conduction band of the secondfloating gate 203 b, there is requirement for applying the energy ofgetting over the energy band gap of the tunnel oxide layer 202. Thisenergy is obtained when the electrons are accelerated with the voltageapplied to the drain region D. In this process, the electrons remainingin the channel region of the semiconductor substrate 201 get over thetunnel oxide layer 202, and then the electrons are transferred to theconduction band of the second floating gate 203 b.

As described above, the second floating gate 203 b is formed of thematerial having the energy band gap (Eg) higher than that of the siliconSi and lower than that of the silicon oxide SiO₂, and the secondfloating gate 203 b is in contact with the first floating gate 203 a ofthe polysilicon material. As a result, the electrons remaining in theconduction band of the second floating gate 203 b are transferred to theconduction band of the first floating gate 203 a which is more stable.

The process of transferring the electrons of the second floating gate203 b to the first floating gate 203 a will be described with referenceto FIG. 6. That is, in the structure of the passivation layer/the secondfloating gate/the first floating gate/the passivation layer, theelectrons remaining the conduction band (Ec) of the second floating gate203 b are transferred to the conduction band (Ec) of the first floatinggate 203 a, wherein the energy band gap in the conduction band of thefirst floating gate 203 a is lower than the energy band gap in theconduction band of the second floating gate 203 b. Thereafter, theelectrons injected to the second floating gate 203 b by the tunnel oxidelayer 202 are transferred to a potential well of the first floating gate203 a. Accordingly, the programming method of the flash memory deviceaccording to the second embodiment of the present invention iscompleted.

In the programming method of the flash memory device according to thesecond embodiment of the present invention, the electrons transferredfrom the source region S are changed to the hot electrons in the channelregion adjacent to the drain region D. Thereafter, the hot electrons getover the potential barrier of the tunnel oxide layer 202, and then aretransferred to the floating gate. In this respect, this process of theprogramming method according to the second embodiment of the presentinvention is very similar to the related art process of the programmingmethod. However, in the programming method of the flash memory deviceaccording to the second embodiment of the present invention, thefloating gate is comprised of the two materials (the first floating gate203 a and the second floating gate 203 b) having the different levels ofthe energy band gap. Also, the energy band gap of the second floatinggate 203 b is higher than the energy band gap of the first floating gate203 a, whereby the electrons injected to the second floating gate 203 bare spontaneously transferred to the first floating gate 203 a.Accordingly, like the related art, trap sites generate in the interfaceand the inside of the tunnel oxide layer 202 below the second floatinggate 203 b having the electrons injected thereto. However, the electronsinjected to the second floating gate 203 b are transferred to the firstfloating gate 203 a having the lower energy band gap, whereby theelectrons injected by the trap sites are not discharged. To dischargethe electrons stored in the first floating gate 203 a to thesemiconductor substrate, it is necessary to pass by the second floatinggate 203 b having the higher energy band gap than that of the firstfloating gate 203 a. That is, as long as the voltage is not applied tothe first floating gate 203 a from the external, the electrons arestored in the first floating gate 203 a, thereby maintaining the appliedthreshold voltage stably.

An erasing method of the flash memory device according to the secondembodiment of the present invention will be described as follows, whichis progressed in the similar process to that of the programming method.

FIG. 7 shows an exemplary view of explaining energy band and transfer ofholes in the semiconductor substrate/the tunnel oxide layer/the secondfloating gate/the dielectric layer/the control gate along II-II′ of FIG.4. FIG. 8 shows an exemplary view of explaining energy band and transferof holes in the passivation layer/the second floating gate/the firstfloating gate/the passivation layer along II-II′ of FIG. 4.

To erase the programmed data, a negative (−) voltage is applied to thecontrol gate (Vg) 205, and a positive (+) voltage is applied to thedrain region (Vd) D. Also, the source region (Vs) S and thesemiconductor substrate (Vsub) 201 are simultaneously grounded orfloated. At this time, it is preferable to provide the voltages appliedto the control gate 205 and the drain region D in the conditions ofgenerating a great amount of hot hole injections.

When the voltages are applied in this conditions, holes generated in thedepletion area 206 of the drain region D, as shown in FIG. 7, areinjected to a valence band of the second floating gate 203 b by thetunnel oxide layer 202. Then, as shown in FIG. 8, the holes injected tothe valence band of the second floating gate 203 b are transferred tothe valence band of the first floating gate 203 a, wherein the energyband gap in the valence band of the first floating gate is lower thanthe energy band gap in the valence band of the second floating gate. Theholes transferred to the valence band of the first floating gate 203 aare recombined with the electrons injected to the conduction band of thefirst floating gate 203 a, whereby the threshold voltage lowers.Accordingly, the electrons stored in the first floating gate 203 a areremoved, so that the flash memory device is maintained in the erasingstate.

In the same way as the programming method, the erasing method also hasthe holes injected by the second floating gate 203 b, wherein theinjected holes are transferred to the first floating gate 203 a havingthe more stable energy level. Accordingly, it is possible to prevent theproblems generated by the trap sites formed in the interface and theinside of the tunnel oxide layer 202 below the second floating gate 203b.

THIRD EMBODIMENT

A flash memory device according to the third embodiment of the presentinvention will be described with reference to FIG. 9. FIG. 9 shows across sectional view of a flash memory device according to the thirdembodiment of the present invention. In the flash memory deviceaccording to the third embodiment of the present invention, as shown inFIG. 9, a tunnel oxide layer 402, a floating gate 403, a dielectriclayer 404 and a control gate 405 are sequentially deposited in an activearea of a p-type semiconductor substrate 401 defined by a deviceisolation layer (not shown).

The floating gate 403 is comprised of a first floating gate 403 a, asecond floating gate 403 b and a third floating gate 403 c, wherein thefirst floating gate 403 a is positioned in the center between the secondfloating gate 403 b and the third floating gate 403 c. At this time, thefirst floating gate 403 a is formed at a width corresponding to that ofthe control gate 405.

Then, n-type impurity ions are implanted to the semiconductor substrate401 at both sides of the first floating gate 403 a/the control gate 405,thereby forming a source region S and a drain region D. Accordingly, thesecond floating gate 403 b and the third floating gate 403 c, being incontact with the first floating gate 403 a, are formed on the tunneloxide layer 402 of the source region S or the drain region D. The tunneloxide layer 402 is extended toward the source region S and the drainregion D at a predetermined degree. Also, although not shown, apassivation layer is formed on an entire surface of the semiconductorsubstrate 401 including the control gate 405.

The dielectric layer 404 may be formed in a structure of oxidelayer-nitride layer-oxide layer. Also, the control gate 405 may beformed of polysilicon to which n-type impurity ions are implanted. Atthis time, the width in each of the second and third floating gates 403b and 403 c is not limited, which is formed at a predetermined minimumwidth to apply a bias-voltage thereto, and not to have effects onspacers and silicide formed in the source region S or the drain regionD.

Also, the first floating gate 403 a is formed of polysilicon. The secondfloating gate 403 b and the third floating gate 403 c are formed of amaterial having an energy band (Eg) higher than that of silicon Si(Eg-1.1 eV) of the semiconductor substrate 401 or the first floatinggate 203 a, and lower than that of oxide layer SiO₂ of the dielectriclayer 404 being in contact with the first floating gate 403 a. Indetail, the second floating gate 403 b and the third floating gate 403 cmay be formed of the same material as that explained in the firstembodiment of the present invention. Then, impurity ions havingdifferent conductive types are implanted to the respective second andthird floating gates 403 b and 403 c. For example, n-type impurity ionsare implanted to the second floating gate 403 b, and p-type impurityions are implanted to the third floating gate 403 c.

Programming and erasing methods using the flash memory device accordingto the third embodiment of the present invention will be described asfollows.

FIG. 10 shows an exemplary view of explaining energy band and transferof electrons in the passivation layer/the second floating gate/the firstfloating gate/the third floating gate/the passivation layer alongIII-III′ of FIG. 9. FIG. 11 shows an exemplary view of explaining energyband and transfer of holes in the passivation layer/the second floatinggate/the first floating gate/the third floating gate/the passivationlayer along III-III′ of FIG. 9.

The programming method using the flash memory device according to thethird embodiment of the present invention is very similar to theprogramming method using the flash memory device according to the firstembodiment of the present invention. First, a positive (+) voltage (Vg)is applied to the control gate 405, and a ground voltage or a negative(−) voltage is applied to the second floating gate (Vf2) 403 b.Simultaneously, the source region (Vs) S, the drain region (Vd) D, thesemiconductor substrate (Vsub) 401 and the third floating gate (Vf3) 403c are floated together.

When the voltage is applied in this conditions, electrons generate dueto the bias-voltage applied to the second floating gate 403 b since thesecond floating gate 403 b is doped with the n-type impurity ions. Then,as shown in FIG. 10, the electrons are transferred to a conduction band(Ec) of the first floating gate 403 a having a more stable energypotential that that of the second floating gate 403 b. At this time, anelectric field applied to the control gate 405 extends to the secondfloating gate 403 b by the first floating gate 403 a, so that electronsinjected from the second floating gate 403 b are stored in a potentialwell of the first floating gate 403 a, thereby increasing a thresholdvoltage.

The erasing method using the flash memory device according to the thirdembodiment of the present invention will be described with reference toFIG. 11. First, to erase the programmed data, a ground voltage or anegative (−) voltage is applied to the control gate 405, and a positive(+) voltage is applied to the third floating gate 403 c. Simultaneously,the source region S/the drain region D, the semiconductor substrate 401and the second floating gate 403 b are floated together. At this time,it is preferable to provide the voltages applied to the control gate 405and the third floating gate 403 c in the conditions of generating agreat amount of direct injections.

When the voltages are applied in this conditions, holes generate due tothe voltage applied to the third floating gate 403 c since the p-typeimpurity ions are doped to the third floating gate 403 c. Then, as shownin FIG. 11, the generated holes are transferred to a valence band (Ev)of the first floating gate 403 a having the more stable energy potentialthan that of the third floating gate 403 c. At this time, as theelectric field applied to the control gate 405 extends to the thirdfloating gate 403 c by the first floating gate 403 a, the holes injectedfrom the third floating gate 403 c are transferred to the valence band(Ev) of the first floating gate 403 a.

After that, the holes transferred to the valence band (Ev) of the firstfloating gate 403 a are recombined with the electrons injected to theconduction band (Ec) of the first floating gate 403 a according to theprogram, whereby the threshold voltage lowers. Accordingly, theelectrons transferred from the second floating gate 403 b to the firstfloating gate 403 a are removed, so that the flash memory device ismaintained in the erasing state.

As mentioned above, the flash memory device and the programming anderasing methods using the same have the following advantages.

In the flash memory device according to the present invention, thefloating gate may be formed of the first floating gate/the secondfloating gate, or the first floating gate/the second floating gate/thethird floating gate. Herein, the energy band gap of the first floatinggate is lower than the energy band gap of the second floating gate/thethird floating gate. Also, the impurity ions are previously implanted tothe second floating gate and the third floating gate. In this state, thevoltages are applied to the second floating gate and the third floatinggate, whereby the electrons or the holes generate, and the generatedelectrons or holes are transferred to the first floating gate having themore stable energy potential. As a result, unlike the related art havingthe problem of the tunnel oxide layer damaged by the hot electroninjection or the hot hole injection, the programming and erasing methodsof the flash memory device according to the present invention preventthe damage of the tunnel oxide layer. Accordingly, it is possible toovercome the problem of leakage current generated by the trap site,thereby maintaining the stable threshold voltage on programming anderasing.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A flash memory device comprising: a first conductive typesemiconductor substrate defined as a field area and an active area; atunnel oxide layer on the active area of the first conductive typesemiconductor substrate; a first floating gate formed on the tunneloxide layer; a second floating gate formed on the tunnel oxide layer,wherein a lateral sidewall of the second floating gate is in contactwith a lateral sidewall of the first floating gate; a dielectric layeron the first floating gate; a control gate on the dielectric layer; andsecond conductive type source/drain regions in the active area of thefirst conductive type semiconductor substrate at both sides of the firstfloating gate.
 2. The flash memory device of claim 1, wherein thedielectric layer is formed in a structure of oxide layer-nitridelayer-oxide layer.
 3. The flash memory device of claim 1, wherein thefirst floating gate and the control gate are formed of polysiliconhaving second conductive type impurity ions implanted thereto.
 4. Theflash memory device of claim 1, wherein the first floating gate has thesame width as the control gate.
 5. The flash memory device of claim 1,wherein the second floating gate is formed above the source/drainregions.
 6. The flash memory device of claim 1, wherein the secondfloating gate has an energy band gap higher than that of the firstconductive type semiconductor substrate and lower than that of thedielectric layer.